Low jitter high phase resolution PLL-based timing recovery system

ABSTRACT

A low jitter, high phase resolution phase lock loop incorporating a ring oscillator-type VCO is designed and constructed to operate at a characteristic frequency M times higher than a required output clock frequency. Multi-phase output signals are taken from the VCO and selected through a Gray code MUX, prior to being divided down to the output clock frequency by a divide-by-M frequency divider circuit. Operating the VCO at frequencies in excess of the output clock frequency, allows jitter to be averaged across a timing cycle M and further allows a reduction in the number of output phase taps, by a scale factor M, without reducing the phase resolution or granularity of the output signal.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is related to co-pending provisional application Ser.No. 60/110,557, filed Dec. 7, 1998, entitled “LOW JITTER HIGH PHASERESOLUTION PLL FOR GIGABIT ETHERNET”, commonly owned by the Assignee ofthe present invention, the entire contents of which are expresslyincorporated herein by reference.

FIELD OF THE INVENTION

The present invention is directed to high speed timing recovery systemsand, more particularly, to a low jitter, high phase resolution timingrecovery system.

BACKGROUND OF THE INVENTION

The past several years have witnessed a dramatic increase in thecapabilities of high-speed, high-density broadband data communicationsystems. Pertinent such systems range anywhere from broadcast orcablecast HDTV systems, local area and wide area (LAN, WAN) systems formultimedia, fiber to the home (FTTH) applications and board-to-boardinterconnections in exchange systems and computers.

In any one of the foregoing applications, it should be noted thatbidirectional data communication is in digital form and, accordingly,clock and data recovery circuitry is a key component to the efficientfunctioning of modern data communications systems. The ability toregenerate clock information from binary data is an inherent advantagein processing information digitally, as opposed to processing suchinformation in its analog form. However, in order that an intelligentsignal be correctly reconstructed at a receiver, binary data must beregenerated with the fewest possible number of bit errors, requiringreceive data to be sampled at an optimum sample rate and at an optimalinstance of time, i.e., accurately with respect to both frequency andphase. Given the bandwidth constraints imposed on most modern datacommunications systems, it is generally impractical to transmit samplingclock information separate from a transmitted datastream. Timinginformation is consequently derived from the incoming transmitted datasignal itself. Extraction of an implicit timing signal is generallytermed timing recovery (or clock recovery) in its functional role ingeneral digital receiver technology, and is traditionally performed bysome form of a phase-lock-loop system.

Also pertinent to binary data regeneration, is the understanding thatnoise corruption of narrowband signals represents a common situation incommunication systems. Noise corruption occurs, for example, insatellite transceivers where intelligence signals, weak with respect tonoise components, must be detected by coherent demodulation. In order toachieve a high signal-to-noise ratio (SNR), the noise components arounda carrier must be suppressed, implying the need for a narrow bandfilter. However, in most applications, the required filter bandwidth isseveral orders of magnitude smaller than typical carriers, therebydemanding relatively large filter quality factors (Qs). Aphase-lock-loop (PLL) is able to operate as a narrow band filter with anextremely high Q.

In many applications which require multi-phase sampling, such as clockand data recovery, frequency synthesis, clock synchronization, and thelike, PLL systems commonly employ ring oscillators, either single-endedor differential, as a frequency and phase generation circuit (voltagecontrolled oscillator or VCO). In many such applications, clock signalsare generated to drive mixers or sampling circuits in which the randomvariation of the sampling instant (jitter) is a critical performanceparameter. In certain applications, the frequency domain equivalent ofjitter (termed phase noise) is also important. Jitter can arise frommany sources, including inadvertent injection of signals from otherparts of a circuit through the power supply. The inherent thermal and/orshot noise of the active and passive devices that make up a VCO cell,and, particularly, the sub-harmonic frequencies of the clock signalitself mixing into the desired output signal.

This last becomes an important design parameter when in it is recognizedthat modern digital clock recovery systems often require multiple clockphases to be provided at a single frequency in order that a clockrecovery system might select the clock phase which best matches theparticular phase of an incoming signal. The more clock phases available,i.e., the higher the phase resolution, the more precisely an incomingsignal can be sampled and the better the overall system performance.

However, it is well recognized that in a VCO design with multiple clockphase outputs, the opportunities for random variation in the triggeringedges, due to inter-stage interaction, RMS voltage noise, cycle-to-cyclejitter, and the like increases. Thus, jitter increases (in a complexrelationship) with the number of phase taps taken from a multi-phase VCOsystem. In addition, for large numbers of clock phases produced by aVCO, it becomes difficult to design a VCO which does not exhibitmultiple modes of oscillation. Thus, even though ring oscillators havebeen proposed as suitable candidates for implementation as low-noisevoltage-controlled oscillators in high-performance PLL systems, theirimplementation has been limited because of their generally characterized“high” phase noise.

Accordingly, there is a need for PLL systems that are able to providemulti-phase output signals with a high phase resolution, and with lowjitter and low phase noise characteristics.

SUMMARY OF THE INVENTION

A high-speed, low-jitter, high phase resolution PLL circuit includes adetector for comparing a phase or frequency characteristic of an inputsignal, such as a reference clock signal, to a phase or frequencycharacteristic of a timing reference signal. A timing reference signalgenerator, such as a VCO, is connected in feedback fashion in order toprovide a timing reference signal to the detector. The timing referencesignal generator is operatively configured to oscillate and therebyproduce an output signal at a characteristic frequency which is anintegral multiple of a desired output clock frequency. Frequency dividercircuitry is provided and is coupled to receive the output signal fromthe VCO and reduce its characteristic frequency to a desired outputclock frequency.

The PLL circuit further includes a loop filter coupled between thephase/frequency detector and the VCO, and develops a control voltage forcontrolling the operational frequency of the VCO. VCO is constructed tooutput multi-phase signals, with each phase signal oscillating at thecharacteristic frequency of the VCO, and each phase signal characterizedby a phase relationship depending on a delay characteristic of acomponent delay cell making up the VCO. The number of phases representedby the multi-phase output signals are reduced by a scale factor M fromthe number of phases characteristically produced by a timing referencesignal generator operating at a characteristic frequency substantiallyequal to the desired output clock frequency.

The PLL circuit further includes a phase select MUX, the phase selectMUX selecting between and among the multi-phase signals in order todefine a respective one of the multi-phase signals as the output clocksignal. The phase select MUX is a Gray code MUX, the MUX selectingbetween and among the multi-phase signals in accordance with a phasecontrol word, the phase control word changing states in accordance witha Gray code sequence. The phase control word has a characteristic bitwidth J, where J has a value mathematically dependent on the frequencyscale factor M.

In a further aspect, the PLL circuit according to the invention includesfrequency divider circuitry disposed between the VCO and thephase/frequency detector and frequency divider circuitry disposedbetween the MUX and an output. The first frequency divider circuitrydivides the output signal of the VCO by a scale factor (N×M) in order todevelop a frequency characteristic which is provided to the detector forthe comparison with the frequency characteristic of an input signal. Thesecond frequency divider circuit divides the output signal of the VCO(and thus the MUX) by a scale factor M in order to develop an outputclock signal at a desired frequency. Timing jitter and phase noise isthus averaged over a timing cycle having a scale factor of M, whilephase resolution granularity is retained.

DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will be more fully understood when considered with respect tothe following detailed description, appended claims, and accompanyingdrawings, wherein:

FIG. 1 is a semi-schematic, simplified block diagram of a high-speedphase-lock-loop system, configured to operate at a frequency M timeshigher than a required output clock frequency, in accordance with thepresent invention;

FIG. 2 is a semi-schematic, simplified block diagram of a multi-phasephase-lock-loop system, configured in accordance with the prior art;

FIG. 3 is a semi-schematic, simplified block diagram of a PLL, includinga multi-phase VCO;

FIG. 4 is a series of waveform diagrams illustrating the phaseresolution relationship between the PLL of FIG. 2 and the multi-phasePLL according to the invention of FIG. 1;

FIG. 5 is a simplified, semi-schematic block diagram of a divide-by-Mmodel which illustrates VCO jitter averaged over M cycles; and

FIG. 6 is a simplified, semi-schematic block diagram of a Gray code MUXsuitable for implementation in a PLL according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, a low jitter, high phaseresolution PLL employs a voltage controlled oscillator (VCO) whichoperates at a characteristic frequency M times higher than thecharacteristic frequency typically required for an output clock signal.Operating a multi-phase VCO at such a higher output frequency reducesthe number of output phases which must be taken from the VCO by the samescale factor M. In addition to reducing the number of VCO output phases,the physical size of selection circuitry, such as a phase control MUX isalso reduced by the scale factor M, while the number of phase controllines, controlling operation of the phase control MUX, are also able tobe reduced by a scale factor related to the scale factor M.

Since the number of VCO phase stages are able to be reduced, powersupply and substrate noise injection is reduced as a consequence,resulting in a lower jitter VCO design. Because the number of outputphases taken from a multi-phase VCO is reduced, the possibility ofmulti-mode oscillation is also reduced.

A design for a low jitter, high phase resolution PLL is illustrated insimplified, semi-schematic block diagram form in FIG. 1.

Phase-lock-loops operate to compare the frequency and/or phase of anincoming serial datastream to a periodic reference clock signalgenerated by an oscillator circuit, and further operate to adjust theoperational frequency and phase characteristics of the oscillator untilits output stream is “locked” in both frequency and phase to a datasignal (or alternatively, to a reference clock signal). An output clockis thereby established which, in turn, controls operation of a decisioncircuit which regenerates (or retimes) the data signal. Thephase-lock-loop suitably includes a phase detector 10 whose output iscoupled to a charge pump circuit 12, operatively connected, in turn, toa loop filter 13 and a voltage controlled oscillator (or VCO) 14.

A reference clock or data signal is received at a data input of a phasedetector 10, in which the currents of the clock's rising edge (itsphase) is compared in time to the occurrence of a rising edge (thephase) of an output signal of the VCO 14. According to convention, thephase detector 10 provides pump signals to the charge pump 12 dependingon whether the data stream phase leads or lags the phase of a clocksignal derived from the VCO 14. A control voltage is developed which isused to control the operational frequency of the VCO 14. The sign of theVCO control voltage variation depends on whether the phase of thedatastream leads or lags the phase of the VCO output and the magnitudeof the VCO control voltage is a function of the extent of the phase leador phase lag.

Thus, the operational frequency of the VCO 14 is increased or decreased,as appropriate, to reduce the phase lead or the phase lag of the inputsof the phase detector 10. The phase-lock-loop thus ensures that the VCOoutput which is used as a timing reference, is locked in phase with theincoming serial datastream, or, in the case of the exemplary embodimentof FIG. 1, with a reference clock signal. Once the PLL is “locked”, thetiming reference signal, i.e., the output clock, is used to controloperation of various decision circuits which define regenerated orretimed data.

Digressing momentarily to the simplified, semi-schematic block diagramof an exemplary embodiment of a multi-phase PLL of FIG. 2, a commondesign implementation of a high phase resolution PLL is to design a VCOwith multiple clock phase outputs, such as a delay cell-based ringoscillator. In the exemplary embodiment of FIG. 2, in which commonfunctional blocks share common reference numerals with the exemplaryembodiment of FIG. 1, the VCO 14 is constructed to operate at acharacteristic frequency, having a frequency multiplication factor, orscale factor N, relative to the frequency of an incoming reference clocksignal. Accordingly, in order to “lock” the VCO's operating frequency tothe frequency of the incoming reference clock signal, a particularoutput phase signal is chosen as representing the VCO output signal anddirected through a divide-by-N circuit 16 prior to being combined withthe incoming reference clock signal in the phase detector 10.

It should be noted that although the VCO 14 is configured to operate ata particular characteristic frequency, N times the ref clock frequency,the implementation of the VCO as a delay cell-based ring oscillator,allows a multiplicity of output signals to be developed, with eachsignal having the same characteristic operational frequency of the VCO,but with each signal differing from another by a characteristic phaserelationship defined by the value of a delay cell, or an integralmultiple thereof. Thus, in the exemplary embodiment of FIG. 2, the VCOis able to produce 2^(K) clock phases, where K is an integer.

A signal bus, carrying the 2^(K) multi-phase signals, is directed to aphase control MUX 18 where signals with particular phase relationshipsare chosen in accordance with a phase control signal, in order to definean output clock signal having the particular frequency and phaserelationship required to operate a decision device such as a DDFS,mixer, demodulator, and the like. It should be noted that the phasecontrol signal, controlling operation of the phase control MUX 18, isimplemented as a bus, where the bus has a signal width of K. Thus,individual ones of the 2^(K) multi-phase signals can be selectivelychosen to define the output clock signal of the PLL.

In practice, the PLL of the exemplary embodiment of FIG. 2 might be usedas a reference source for a high-speed decoder circuit or a high-speedADC. Sampling inaccuracies due to phase offsets are able to becharacterized and written to a register, for example. Phase offset wordswritten to such a register might provide the source for the phasecontrol signals directed to the phase control MUX 18. An observed phaselag or phase lead in an ADC, for example, can be compensated by choosingthe appropriate one of the 2^(K) signals in order to advance or retardthe phase characteristic of the PLL's output clock signal so as tocompensate the observed phase lead or phase lag of the ADC timingwindow.

Returning now to the exemplary embodiment of a PLL system of FIG. 1, theVCO 14 is also configured to operate at a characteristic frequencyhaving a frequency multiplication factor N relative to an inputreference clock signal, and is further implemented to operate at afrequency M times higher than the required output clock frequency, i.e.,operating at a frequency multiplication or scale factor of M×N relativeto the input reference clock. As was the case with the PLL of FIG. 2, aVCO output signal is directed through frequency divider circuitry priorto being combined with an input reference clock signal in a phasedetector 10. Frequency divider circuitry suitably includes a divide-by-Ncircuit 16, as well as a divide-by-M circuit 20. Although the frequencydivider circuits are illustrated as being provided separately in theexemplary embodiment of FIG. 1, it should be understood that this issolely for purposes of ease of illustration. Frequency divider circuitrymay be provided in a single circuit, or multiple circuits, so long asthe various frequency multiplication factors, relative to an incomingreference clock signal, are accommodated.

Because the VCO 14 of the exemplary embodiment of FIG. 1 is operating ata frequency M times higher than the required output clock frequency, thenumber of output phases that the VCO needs to provide may be reduced bya scale factor M. The VCO 14 thus provides 2^(J) output phase signals,where 2^(J)=2^(K)/M. The output phase signals are directed to a phasecontrol MUX 22 where appropriate output phase signals are selected by aphase control signal bus 24, having a signal width J as will bedescribed in greater detail below. Once an appropriate output phasesignal is chosen from phase control MUX 22, an output clock signal isdeveloped by directing the chosen signal through a divide-by-M frequencydivider circuit 26. The VCO 14 and phase control MUX 22, function todefine an output signal having a particular desired phase state, whilethe divide-by-M frequency divider circuit 26 functions to provide theparticular phase date signal at the appropriate output frequency.

FIG. 3 illustrates, in simplified, semi-schematic block diagram form, anexemplary PLL with a VCO portion 14 implemented as a delay cell-basedring oscillator including multiple clock phase outputs. As was the casewith the exemplary embodiments of FIGS. 1 and 2, the VCO output iscombined with a reference clock signal, for example, in a phase detector10 in order to derive a pump signal directed to charge pump 12. Thecharge pump 12 develops pump up and pump down signals through loopfilter 13 in order to derive a control voltage which controlsoperational speed of the VCO 14 by controlling the delay of each of itscomponent delay stages. Multi-phase sampling is performed by taking anoutput from the VCO 14 at particular selected ones of its varioussequentially disposed delay stages. Each of the outputs will develop asignal having the characteristic oscillation frequency of the VCO, buthaving a phase relationship with preceding and following signals, withthe phase relationship depending on the amount of delay present in thedelay stage or stages between each successive output.

The particular relationship between output phase and output frequency isdepicted in the waveform diagrams of FIG. 4, with the uppermost set ofwaveform diagrams representing output signals developed by a VCOoperating at a conventional frequency N times the reference clockfrequency. Four waveforms are illustrated in the upper portion of theexemplary embodiment of FIG. 4, with each waveform representing a signaltaken from the VCO every 90° of phase.

The lower portion of the exemplary waveform diagram of FIG. 4 representsa multiplicity of output signals developed by a VCO, operating inaccordance with principles of the present invention, in which thefrequency of the VCO is M times higher than the frequency of the VCOrepresented in the upper portion, i.e., in the case of the exemplaryembodiment of FIG. 4, the high-speed VCO is operating at 4 times therequired output clock frequency. As will be evident from the waveformdiagrams of FIG. 4, a high-speed VCO is able to develop a significantlylarger number of transition edges within an output clock period than therelatively lower speed VCO such as might be represented by FIG. 2.Indeed, the high-speed VCO, such as might be represented by theexemplary embodiment of FIG. 1, is able to develop 17 transition edges,representing 17 quasi-phase states within the same output clock periodin which 5 quasi-phase states are developed by the relatively lowerspeed VCO of FIG. 2.

In practical terms, this allows the number of physical output phasetaps, from the VCO, to be reduced by a scale factor M (in the presentcase a scale factor of 4) while retaining the same phase resolutiongranularity of the relatively lower speed system.

Further, and in accordance with principles of the present invention, thephysical size of the phase control selection MUX (22 of FIG. 1) is alsoable to be reduced by scale factor M, with the number of phase controllines represented mathematically by a value J=Kx (log2/logM). It shouldbe further recognized that the scale factor M should be chosen as apower of 2.

Since the number of stages in the VCO are able to be reduced, its powersupply noise and substrate noise injection characteristics can also bereduced, resulting in a lower jitter VCO design. In the particular casewhere the VCO output frequency is M times the desired output clockfrequency, it will be evident that VCO jitter is averaged over M cycles,as is depicted in the generalized block diagram of FIG. 5. In FIG. 5,each of the delay stages of an exemplary ring oscillator-type circuit isrepresented by a delay element Z⁻¹ 30 with each delay element's jittercontribution represented by a “delta timing” value ΔT_(VCO). Classicaltheory suggests that adding a frequency divider between a VCO and anoutput node or phase detector, results in increasing VCO jitter as thedivision ratio M increases. Measured data, however, contradicts thisclassical prediction. A divide-by-M circuit (20 and 26 of FIG. 1) can bemodeled by an M-bit-long shift register which is equivalent to cascadingM-1 unit delay blocks 30 in the Z-domain. Since integration of the VCOperiod change AT is performed on a cycle basis, the jitter at the outputof the divide-by-M circuit (32 of FIG. 5) can be expressed asΔT_(M)=ΔT_(VCO)/M. Thus, it should be clear that a divide-by-M circuitimplements moving average filter, with little or no averaging performedat lower frequencies.

The architecture of the low jitter high-phase resolution PLL describedin connection with the exemplary embodiment of FIG. 1, places fewconstraints on the system level implementation of a digital clockrecovery system. However, as will be well understood by those havingskill in the art, phase control must be incremented or decrementedsequentially one step at a time. The random phase access of theexemplary PLL system described in connection with FIG. 2, is foregone infavor of the low jitter characteristics and high phase resolution of thePLL system according to the present invention. As those having skill inthe art will appreciate, this is generally not an issue in communicationsystems which have a constant carrier link.

A further aspect of the present invention, which reduces substrate noiseinjection characteristics and reduces the “glitch” potential of theoutput clock when a phase control signal is changed in the phase controlMUX (22 of FIG. 1) includes a Gray code implementation to the phasecontrol MUX which promotes smooth and orderly transitions between phasestates of the VCO. Such a Gray code MUX is depicted in semi-schematicsimplified block diagram form the exemplary embodiment of FIG. 6. Inparticular, the MUX is implemented as a three bank cascade, with thefirst input bank constructed of four parallel MUX elements 34, 36, 38and 40, with each of the input MUX elements configured to receive andselect between two input signals, with each input signal representing aparticularly phased signal, each signal being differential and eachsignal representing a 180° phase relationship. The choice between theMUX bank's first or second inputs(input 0 or input 1) is made by themost significant bit (the embodiment of FIG. 6 bit-2) of the phasecontrol word, having characteristic width J (in the exemplary embodimentof FIG. 6, having characteristic width of three bits). Once the inputstate of the input MUX bank is selected, the signals are directed to anintermediate MUX bank, including two parallel MUX elements 42 and 44,each configured to receive two input signals at their binary definedinputs (input 1 or input 0). The choice between binary input states ofthe intermediate MUX bank is made by the next most significant bit ismade by the next most significant bit (bit-1) of the phase control wordand the resulting signals are directed to the binary defined inputs ofan output MUX 46. The final choice of the particular phase of the signalto be developed as an output clock signal is made by the least mostsignificant bit (bit-0) of the phase control word.

Thus, it will be understood that the Gray code MUX, in accordance withthe present invention, allows for smooth transitions between selectedoutput phases, by virtue of the operation of a Gray code sequence. In aGray code sequence, only a single bit is allowed to change from aprevious state to a present state and from a present state to asubsequent state. Thus, there is only a limited opportunity for theswitch logic circuitry of the phase control MUX to introduce noise and“glitch” opportunities into operation of the system.

In order to further improve system speed and minimize the system'ssensitivity to power supply and substrate noise, and further reduce thesystem's susceptibility to “glitches” and jitter, the system's switchlogic circuitry, such as the MUX and divide-by-M frequency dividercircuitry, are implemented in current-mode-logic. However, it will beevident to one having skill in the art that the invention may likewisebe suitably implemented in various other semiconductor technologies,such as bipolar, bi-CMOS, and the like, and may also be suitablyimplemented in various other logical forms, such as ECL, VML, and thelike. Moreover, the various circuit elements according to the inventionmay be constructed from discrete components or as a monolithicintegrated circuit, depending upon the particular needs of acommunication system, or the desires of a system designer. Voltagecontrolled oscillator circuitry may be implemented as a ringoscillator-type VCO and might be constructed of either single ended ordifferential unit delay cells.

It will thus be recognized by those skilled in the art that variousmodifications may be made to the illustrated and other embodiments ofthe invention described above, without departing from the broadinventive scope thereof. It will be understood, therefore, that theinvention is not limited to the particular embodiments or arrangementsdisclosed, but is rather intended to cover any changes, adaptations ormodifications which are within the scope and spirit of the invention asdefined by the appended claims.

What is claimed is:
 1. A phase lock loop for providing an output signalwith a desired characteristic frequency that is an integer, N, multipleof a characteristic frequency of an input signal, said phase lock loopcomprising: a voltage controlled oscillator generating one or moreoscillator signals, wherein each of the one or more oscillator signalsare associated with M phases of the desired characteristic frequency ofthe output signal; a phase detector for comparing a phase or thefrequency characteristic of the input signal to a phase or frequencycharacteristic of a particular one of the one or more oscillator signalsto the input signal; and a Gray code multiplexer for selecting aparticular one of the one or more oscillator signals based on a GrayCode, said Gray code multiplexer further comprising: a first multiplexerreceiving two of the one or more oscillator signals, wherein the two ofthe one or more oscillator signals have approximately a 180 degree phaseshift with respect to each other; and a second multiplexer receivinganother two of the one or more oscillator signals, wherein the anothertwo of the one or more oscillator signals have approximately a 180degree phase shift with respect to each other.
 2. The phase lock loop ofclaim 1, further comprising: a divider circuit for reducing acharacteristic frequency of the selected one of the one or moreoscillator signals to the desired characteristic frequency.
 3. The phaselock loop of claim 2, further comprising: one or more divider circuitsfor reducing the characteristic frequency of at least one of the one ormore oscillator signals.
 4. The phase lock loop of claim 3, wherein theone or more divider circuits reduce the frequency characteristic of theat least one of the one or more oscillator signals by an integer, M,factor and an integer, N, factor.
 5. The phase lock loop of claim 1,further comprising: a loop filter for increasing or decreasing acharacteristic frequency of the voltage controlled oscillator signalsbased on the comparison of the phase or frequency characteristic of theinput signal to the phase or frequency characteristic of the particularone of the one or more reference clock signals by the detector.